Road vehicle wheel and pneumatic tyre assemblies are mounted to a vehicle for rotation about an axis, the axis typically being parallel to an axle on which wheel and tyre assembly is mounted. Since the wheel and tyre assembly is a solid body, its moment of inertia is a second rank tensor and unless the axis of rotation of the assembly is parallel to one or its principle axes of inertia rotation of the wheel will result in out of balance forces. This can cause the wheel to wobble and can also lead to excessive wear of certain parts of the vehicle, especially wheel bearings. If either of the front wheels of an automobile are out of balance they can cause the steering wheel of the vehicle to vibrate.
For this reason, wheel and tyre assemblies are balanced, to ensure that the mass of the wheel and tyre assembly is distributed evenly around axis of rotation of the assembly so that the axis of rotation of the wheel coincides with one of the principle axes of inertia of the assembly.
Typically, vehicle wheels are balanced by affixing balance weights to the inside of the wheel rim. After a tyre has been fitted to the wheel the resulting wheel and tyre assembly is rebalanced by affixing balance weights to the outside flange of the rim of the wheel in order to compensate for mass unevenly distributed in the tyre.
Other known methods of balancing a wheel and tyre assembly employ a mass that has some freedom to move so as to dynamically balance the assembly. U.S. Pat. No. 4,269,451 and US2007/0126279 disclose the use of one or more tubes which are at least partially filled with fluid and spherical masses and which extend around at least a part of the circumference of the wheel assembly. In use, as the wheel assembly rotates, centrifugal forces distribute the mass in the tube so as to balance the assembly.
In common with fixed weights, the tubes of US2007/0126279 are shown mounted to either the inside of the wheel rim, or to the outside flange of the wheel. A problem with affixing weights to the inside of a wheel rim is that they limit the amount of space remaining within the wheel rim in which to locate brakes. With increases in vehicle performance there is a need to increase the size of brake assemblies, particularly brake discs, which in outboard brake installations are usually located within the rim of the vehicle wheel.
The arrangement disclosed in U.S. Pat. No. 4,269,451 overcomes this problem by locating the moveable weights within the tyre. The arrangement where an annular tube is mounted in rings secured to the radially inner surface of the tread portion of the tyre is complex to install. Whilst it is disclosed that the tube may be supported in the tyre by way of a snug fit it is not explained how this would in practice be achieved and in any event is not recognised as a preferred approach since radial movement of the tube is then prevented.
It is an object of embodiments of the present invention to provide an improved, alternative balancer for a wheel and tyre assembly which at least partially overcomes or alleviates the above problems.